1. Field of the Invention
This invention relates generally to a small economical current sensor that can be used in an inexpensive Arc Fault Circuit Interrupter (AFCI) that may also provide Ground Fault Protection, and more particularly to an arc fault current sensor which has common and single mode to ground noise rejection and bandpass filtering features, and which also may act as a grounded neutral feedback transmitter, or ground fault detector, when used in a combination device.
2. Description of the Prior Art
Arc detection is an enhancement to thermal and magnetic overload detection typically used in circuit breakers, which may not otherwise detect and respond to arc faults. A number of devices for detecting arc faults and methods of detection have been used in the past. These include E and B field arc sensors, detecting the magnitude of the rate of change of current signals when an arc fault occurs, the use of non-overlapping band pass filters to detect white noise characteristic of arcs, and detecting the disappearance of signals indicating the presence of arcs near zero current crossings. While some of these techniques are more or less effective, they require relatively sophisticated arc sensors and circuits. Heretofore, most arc detection circuits have been incorporated in circuit breakers because, among other things, the circuits were too large to fit in a wiring device such as a receptacle.
There is a need for simple economical arc fault detectors that can be included in wiring devices such as receptacles, plugs, or in-line devices, and that offer the same protection as an arc fault detector incorporated in a circuit breaker, but at lower cost.
There is a need for an arc fault detector in wiring devices that can be provided at a reduced cost compared with arc fault detecting circuit breakers which is comparable to the reduction in cost between ground fault interrupting receptacles and ground fault interrupting circuit breakers.
There is the need for a sensor and associated circuitry for an AFCI that is miniaturized. There is also need for a sensor and associated circuitry that effectively senses and allows protection against both arc faults and power line grounded neutral faults.
This invention discloses an Arc Fault detector with a sensor having a winding configuration that rejects common mode, and single mode to ground noise originating from the line side of the device, which may mimic arc fault noise. The sensor also enhances the signals generated from arc fault noise, while at the same time performing the dual function of a grounded neutral transmitter, or of a ground fault detector, which permits a much smaller and less expensive current transformer sensor to be used without sacrificing the detector""s ability to respond to a broad range of arc fault currents. This allows a less expensive and smaller overall circuit which can be constructed to fit into a wiring sized device and which may also permit a dual-purpose arc and ground fault detection circuit.
It is an object of this invention to provide an arc fault circuit interrupter, also known as AFCI, that employs an electrical circuit that is simple enough, inexpensive enough and small enough to be included in wiring devices.
It is another object of this invention to provide an arc fault circuit interrupter that is sensitive to relatively low amplitude series arc faults of at least 5 amps of arc current, typically in series with the load and commonly referred to as Type A faults.
It is another object of this invention to provide an arc fault circuit interrupter that detects parallel or line to line arcs producing currents of 75 amps or more, commonly referred to as Type B arc faults.
It is another object of this invention to provide a ground fault circuit interrupter, also known as a GFCI, that detects power line frequency leakage to ground, typically 60 Hertz, whose current is 5 milliamps or more, or inadvertent neutral connections to ground.
Briefly stated, and in accordance with a presently preferred embodiment of the invention, an arc fault detector for detecting electric power lines includes a current transformer preferably in the shape of a toroid, with a neutral conductor and at least one line conductor passing through the aperture of the toroid. The sensor has two multi-turn windings each formed around a portion of the toroidal core, with one winding adjacent to each of the hot and neutral wires respectively, with both windings connected in series in a way which re-enforces arc fault noise generated by arc faults involving the line and neutral, but which causes signal reduction for noise signals from the line and neutral, or either, to ground. The windings and core are also selected to self resonate at a frequency or over a band of frequencies that excludes power line carrier frequencies but which includes arc fault frequencies. The core may also have a third winding, acting as a grounded neutral transmitter, or ground fault detector, or instead of a third winding, one of the arc fault sensing windings can act as dual function sensor for both.